Expiration dating for Standard and Reference preparations has often been assigned arbitrarily; since references and standards are a critical part of release and research activities of the Center, the characterization of these materials with respect to chemical and immunological stability has begun. Using methods of SDS PAGE, 2-D, and crossed immuno- electrophoresis, Isoelectric Focusing and complementary immunoblotting techniques, Diphtheria and Tetanus toxin and toxoid preparations stored at different temperatures were evaluated qualitatively. Emphasis has been placed on stability of Diphtheria toxin and antitoxin, since a new assay method for Diphtheria Antitoxin is being investigated. This new assay method, consisting of a cell survival assay, has been employed to examine the retention of potency of both reagents. This assay has provided quantitiative data to supplement qualitative analyses that support extension of dating for certain reference preparations.